Endovascular stents are coated frequently with a drug polymer that contains one or more therapeutic compounds within a polymeric matrix to improve the efficacy of the stents. These compounds are eluted from the stent coating to the tissue bed surrounding the implanted stent. The effectiveness of these drugs is generally improved because localized levels of medication may be higher and potentially more successful than orally or intravenously delivered drugs, which are distributed throughout the body rather than concentrated at the location of most need. Drugs released from tailored stent coatings may have controlled, timed-release qualities, eluting their bioactive agents over hours, weeks or even months. A common solvent or a pair of solvents may be used to dissolve drugs and polymers, including copolymers, terpolymers or polymer blends. Then the drug-polymer solution is sprayed or dipped on the stent. Upon drying, the drug-polymer coating is formed on the stent surface.
Polymer matrices containing the compounds must be reliably attached to the stent to control delivery of the pharmaceutical compounds, to maintain high quality during manufacturing of such a stent, and to prevent cracking or flaking of the drug-polymer coating when the stent is deployed. Problems may arise in getting coatings to adhere to stents, particularly stents made of stainless steel. Most coronary stents are made of stainless steel or tantalum and are finished by electrochemical polishing for surface smoothness. A smooth surface is desirable because early research has shown that a stent with a rough surface results in more platelet cell adhesion, thrombus, inflammation, and restenosis than a smoothly polished stent. The smooth surface may pose a challenge to the coating, however. Due to the very different nature of the polymer and the metallic substrate, polymers do not easily adhere to the metallic substrate. If the coating does not adhere well to the metal surface, it may cause problems such as coating delamination, irregular drug release profiles, or embolism caused by broken and detached debris from the coating.
The coating may crack or fall off during assembly, packaging, storage, shipping, preparation and sterilization prior to deployment unless effectively adhered to the stent framework. Degradation of the polymer coating may occur with prolonged exposure to light and air, as the constituents of the drug polymer may oxidize or the molecular chains may scission. Although degradation of the polymer coating is of major concern, it is imperative that the adhesion strength of the coating be greater than the cohesive strength of the polymeric matrix to avoid any loss of the coating.
Polymeric coatings have a tendency to peel or separate from an underlying metallic stent because of low adhesion strength typically found between polymers and metals. Many polymers are non-polar or have limited polarization, reducing their ability to stick to the metal stent framework. Temperature excursions of the coated stent and the difference in thermal expansion coefficients between the metal and the coating may contribute to the fatigue and failure of the bond. Materials that are optimal for drug compatibility and elution may not, in and of themselves, provide sufficient adhesion to a metal substrate. A method to improve the adhesion between a drug-polymer coating and a metallic stent, while retaining the therapeutic characteristics of the drug-polymer stent, would be beneficial. Conventional polymers could be incorporated into the drug-polymer coating. If the adhesive strength of the polymeric coating were improved, a more robust stenting device could be made. The coating profiles may be lower and the stent struts could touch. It is desired to have an adhesion layer or primer that is biocompatible, promotes good adhesion between metals and polymers, is easy to process, and is reliable.
It is an object of this invention, therefore, to provide a drug-coated stent with an effective adhesion layer between the drug polymer and the underlying stent framework, to provide a method for manufacturing a drug-polymer coating on a metallic stent, to provide a system for treating heart disease and other vascular conditions using drug-eluting stents with improved adhesion between the drug polymer and the stent, and to overcome the deficiencies and limitations described above.